Study aim: Determining the effectiveness of mindfulness-based cognitive therapy (MBCT) on alexithymia, self-critical rumination, and psychological flexibility in people with somatic symptom disorder Design: This clinical trial study is a controlled, parallel-group, unblinded, and randomized (sealed envelope) study. It is conducted on 38 individuals with somatic symptom disorder referred to psychotherapy centers located in Isfahan. The sealed envelope method was used for randomization. Settings and conduct: 38 eligible individuals were randomly assigned to two experimental and control groups (19 each). Both groups completed the pre-test questionnaires of emotional alexithymia, self-critical rumination, and psychological flexibility. The experimental group participated in 8 sessions of 90 minutes of mindfulness-based cognitive therapy at the analytical psychotherapy center. The control group did not receive any treatment. Finally, both groups completed the post-test. Participants/Inclusion and exclusion criteria: Inclusion Criteria: Having somatic symptom disorder; Age range between 20 and 50 years; Sufficient physical ability to attend the sessions; Informed consent and willingness to participate in the research process; Minimum literacy level (reading and writing skills) Exclusion Criteria: Receiving any psychological treatment in the control group ; Presence of acute psychological disorders such as depression or anxiety, according to psychiatric history; History of physical illnesses requiring medication or special dietary regimen Intervention groups: 19 eligible individuals who complete research questionnaires as a pre-test but do not receive any treatment. Intervention group: 19 eligible individuals who participated in mindfulness-based cognitive therapy sessions after the pre-test. Main outcome variables: Alexithymia, self-critical rumination, psychological flexibility